1. Field of the Invention
The present invention relates to the recovery of hydrogen and the separation of hydrogen isotopes. In particular, the present invention relates to a hydride absorption/desorption apparatus for the recovery of hydrogen and the separation of the isotopes of hydrogen.
2. Discussion of Background
Processes for the separation of hydrogen isotopes often rely on hydrogen-absorbing materials (hydrides) for the recovery, storage and supply of the isotopes. Hydrides are capable of absorbing large amounts of hydrogen which can then be desorbed under the appropriate temperature and pressure conditions. They are selective in that they only absorb hydrogen, and also differentially absorb the three isotopes of hydrogen (protium, deuterium, and tritium).
When hydrogen contacts a hydride, the temperature of the hydride rises as it absorbs hydrogen in an exothermic reaction. Since the hydrogen equilibrium pressure increases exponentially with increasing temperature, hydrogen absorption decreases with increasing temperature. Absorption ceases when the partial pressure of hydrogen is equal to the equilibrium pressure. Therefore, the hydride must be cooled to maintain the absorption process. To release hydrogen, the reaction is reversed by heating the hydride. The faster the hydride is cooled and heated, the faster the hydrogen is absorbed and released, respectively.
Known hydrides include pure metals (Mg, Ti, V, Nb, Pt, Pd, and so forth), alloys (the La-, Ti-, and Co- alloys, rare earth-Ni alloys), and various hydride-containing compositions. The capacity of a particular material to absorb or release hydrogen depends on the temperature, the external hydrogen gas pressure, and the surface area of the material. To maximize surface area and absorption/desorption efficiency, the hydride is often supplied in the form of small-grained particles or pellets.
Typical hydrogen separation apparatus includes a column at least partially filled with a hydride. A hydrogen-containing gas mixture is flowed through the column to separate hydrogen from the mixture; the column is heated to recover the hydrogen. A plurality of columns, arranged in "series" or "parallel," may be provided to increase the efficiency of the process. For example, channels might be machined into an aluminum or stainless steel block, filled with a hydride, and covered by a plate welded thereto. Hydride-containing columns may be arranged in parallel within a sealed housing, as in the apparatus described by Konishi, et al. (U.S. Pat. No. 4,859,427). Heat is supplied by applying an electric current to heating coils disposed within the housing. Hydrogen or a hydrogen-containing mixture enters the housing through an inlet and portions thereof are diverted to flow through the individual columns.
Known designs of this type generally use straight columns. Since the efficiency of the absorption/desorption process depends in part on how rapidly the column is heated and cooled; the faster the cooling and heating, the higher the efficiency. The cooling and heating rate increases with the column surface area and the heat transfer efficiency between the column surface and the heat transfer medium. The heat transfer efficiency in turn depends on the flow pattern of the heat transfer medium over the external surface of the column. The column surface area is sometimes increased by the use of multiple columns. Designs using multiple columns typically contain a large number of fittings, seams, welds, or couplings. In many cases, it is difficult to examine these to assure the structural strength and integrity of the apparatus. There is a need for an efficient hydrogen isotope separation apparatus having a large column surface area with a minimum of welds or other couplings and a high heat transfer efficiency.